BatchSpawnerBase: Extend connect_to_job for port forwarding approaches.

This allows to use {rport} inside the connect_to_job_cmd.
If this is done, {port} is set to a random_port chosen locally
on the Hub, and {rport} is set to the original remote port.
This is useful e.g. for SSH port forwarding.

If this is used, the {rport} of the notebook is saved into
a new class variable in case it will be needed again.

batchspawner/batchspawner.py

@@ -176,8 +176,16 @@ def _req_keepvars_default(self):
         help="Command to connect to running batch job and forward the port "
              "of the running notebook to the Hub. If empty, direct connectivity is assumed. "
              "Uses self.job_id as {job_id} and the self.port as {port}."
+             "If {rport} is used in this string, it is set to self.port, "
+             "and a new random self.port is chosen locally and used as {port}."
+             "This is useful e.g. for SSH port forwarding."
         ).tag(config=True)
 
+    rport = Integer(0,
+        help="Remote port of notebook, will be set if it differs from the self.port."
+             "This is set by connect_to_job() if needed."
+        )
+
     # Raw output of job submission command unless overridden
     job_id = Unicode()
 
@@ -208,12 +216,19 @@ def cmd_formatted_for_batch(self):
 
     async def connect_to_job(self):
         """This command ensures the port of the singleuser server is reachable from the
-        Batchspawner machine. By default, it does nothing, i.e. direct connectivity
-        is assumed.
+        Batchspawner machine. Only called if connect_to_job_cmd is set.
+        If the template string connect_to_job_cmd contains {rport},
+        a new random self.port is chosen locally (useful e.g. for SSH port forwarding).
         """
         subvars = self.get_req_subvars()
         subvars['job_id'] = self.job_id
-        subvars['port'] = self.port
+        if '{rport}' in self.connect_to_job_cmd:
+            self.rport = self.port
+            self.port = random_port()
+            subvars['rport'] = self.rport
+            subvars['port'] = self.port
+        else:
+            subvars['port'] = self.port
         cmd = ' '.join((format_template(self.exec_prefix, **subvars),
                         format_template(self.connect_to_job_cmd, **subvars)))
         await self.run_background_command(cmd)